1. Field of the Invention
The present invention relates to the field of cryogenic storage containers and in particular to a cryogenic tank which is retrofitted to receive an in-tank submerged pump for pumping the cryogen directly out of the primary storage tank with a minimum of heat leakage into the cryogenic tank.
2. Description of the Prior Art
A cryogenic fluid or cryogen such as liquid nitrogen is a substance which exists in the liquid state only at very low temperatures and consequently has a very low boiling point. Because of this low boiling point, two primary considerations when designing a system for storing and pumping a cryogen are the need for adequate insulation of the storage container to minimize losses of cryogen due to "boiloff", and the need to cool down the pump to the cryogen temperature before pumping.
In order to meet the first criterion, cryogenic tanks rely on good thermal and/or radiation barriers, i.e., insulation, high vacuums between container walls, and construction techniques which minimize the thermal leak paths from the exterior environment into the cryogen. Typical thermal paths in cryogenic storage systems include conduction, convection and radiation between the inner and outer shells, fluid and gas lines which connect the inner shell to the outside, supports for the inner shell of a multi-shell container, and any connection to pumps for pumping the cryogen from the primary storage tank. Due to its mass and its inevitable contact with the cryogen, a pump normally provides a high thermal leak path which in existing systems has lead to unacceptably high losses of cryogen due to boiloff.
The solution to this problem generally adopted in the past has been to locate the pump outside the primary cryogenic storage tank where the pump is normally kept at ambient temperature. However, in order to keep the cryogen in the liquid state while being pumped, the pump must be cooled down to the cryogen temperature before pumping can begin. This therefore introduces a delay in system start-up, as it usually takes at least five to ten minutes to cool down the pump sufficiently. When an auxiliary sump is used, the sump must also be cooled down in order to prepare the system for a pumping operation. Cooling down the pump and sump is wasteful of cryogen since a quantity of the liquid is lost in the cool-down procedure by boiloff. In situations where a start-up delay is unacceptable, the pump must be kept in a standby condition in readiness for immediate operation. The pump must therefore be kept in a cooled-down state by being submerged in the cryogen, either in the primary storage tank or in an auxiliary sump, and high rates of boiloff must be tolerated. The use of auxiliary sumps is common because the heat leak through the pump into the sump is isolated from the main storage tank, and the loss of cryogen can be reduced when standby is not required by shutting off the pump/sump from the main storage tank. Nevertheless, the use of sumps represents a compromise which increases the cost and complexity of cryogenic storage systems.
The assignee of the present invention has solved these problems by devising a design for a built-in submerged pump which can be kept in a continuously cooled-down state in readiness for immediate operation, without excessive losses of cryogen by boiloff due to heat leakage through the pump into the interior of the primary storage container. See, for example, Zwick, "Cryogenic Storage Tank with a Built-in Pump", U.S. Pat. No. 4,472,946, which is expressly incorporated herein by reference.
However, the advantages of the Zwick design are achieved in large part by the design integration of the built-in submerged pump in the cryogenic tank. The cryogenic tank and the built-in pump are thus made as a single design unit, each with features which serve in combination to achieve the objects as a whole. Thus, it cannot be expected that preexisting cryogenic tanks can be utilized in any meaningful way to obtain the advantages of the Zwick design.
Therefore, what is needed is an improvement which allows many or all of the advantages of a cryogenic tank with an integrally built-in design submerged pump to also be achieved in a retrofitted conventionally designed cryogenic tank.